1. Field of the Invention
The present invention relates to a variable valve timing mechanism for changing opening/closing timing of at least an inlet valve or an exhaust valve of an internal combustion engine (hereinafter referred to simply as an "engine") according to engine operating conditions.
2. Description of Related Art
Hitherto, there has been known a vane type variable valve timing mechanism as disclosed in JP-A-8-121123 as a variable valve timing mechanism for driving a camshaft via a timing pulley and a chain sprocket which rotate in synchronism with a crank shaft of an engine to give a phase difference by relative rotation between the timing pulley and the chain sprocket and the camshaft.
The vane type variable valve timing mechanism relatively rotates the crank shaft and the camshaft by rotating vanes by a pressure difference between two hydraulic chambers provided on both sides in each vane in the circumferential direction.
The conventional variable valve timing mechanism supplies oil to respective hydraulic chambers via oil passages provided within the camshaft. However, it may be preferable to supply hydraulic pressure from an end opposite to the camshaft (front side) because of the design condition for the engine when, for example, it is difficult to provide the oil passage within the camshaft.
Another known helical type variable valve timing mechanism is disclosed in JP-A-7-507120 (PCT publication number in Japan). This helical type variable valve timing mechanism supplies hydraulic pressure from the end of the front side. However, the helical type variable valve timing mechanism has such problems that it requires a large number of parts.
Furthermore, in case of the vane type variable valve timing mechanism, a clearance between a vane rotor and a housing member is required because the vane rotor rotates relative to the housing member. It then becomes difficult to control the phase difference between the vane rotor and the housing member with high degree of accuracy when pressure oil within the hydraulic chamber leaks via this clearance. Therefore, it is important to reduce the leak of the oil for the vane type variable valve timing mechanism.